1. Field of the Invention
The present invention relates in general to a marine propulsion system, and more particularly to a marine engine.
2. Description of Related Art
Conventional internal combustion engines typically circulate air within the lubrication system of the engine to enhance lubrication and to extend the life of the lubricant. For this purpose, many internal combustion engines allow some combustion gases, which blow by the piston rings into the crankcase ("blow-by gases"), to circulate within the lubrication system.
Internal combustion engines typically employ a ventilation system to vent the blow-by gas from the lubrication system in order to produce an air flow through the crankcase. Such ventilation systems are common in both outboard motors and inboard-outboard motors.
Prior ventilation systems commonly exhaust the blow-by gas from the lubrication system at the cylinder head and introduce the removed blow-by gas back into the induction system for eventual expulsion through a conventional exhaust system. These systems typically direct the blow-by gas into an intake silencer of an induction system of the engine via a hose, which is passed around the periphery of the engine. In the intake silencer, the blow-by gas often initially flows into a dedicated expansion chamber to diffuse before induction into the engine. The dedicated expansion chamber and an induction passage within the intake silencer commonly are positioned in parallel so that the blow-by gas is drawn into the air flow within the induction passage just before the air enters a fuel charge forming device (e.g., a set of carburetors). Japanese Patent Publication 4-1661 discloses an example of one such prior ventilation system.
Though effective in venting blow-by gas from the crankcase, prior ventilation systems commonly are too large and protrusive, and overly complicated. Consequently, the girth of the engine and protective cowling must be increased, thereby increasing drag on the watercraft.
Prior crankcase ventilation systems also do not evenly distribute the blow-by gas between all cylinders. These systems tend to introduce the blow-by gas at an edge of the airflow stream through the intake silencer and do not facilitate thorough mixing of the air and blow-by gas before induction into the charge forming device. For instance, where the charge forming device comprises aligned carburetors (such as illustrated in Japanese patent publication 4-1661), those carburetors closest to the side of the airstream where the blow-by gas is introduced receive a higher concentration of blow-by gas than do the balance of the carburetors. As a result, the blow-by gas is not evenly distributed between the cylinder, and some cylinders run on a richer air/fuel mixture than others, thus affecting the performance of the engine.